feat: audit log / history timeline on instance detail page

Adds an instance_history table that records every field change:
- createInstance logs a 'created' event
- updateInstance diffs old vs new and logs one row per changed field
  (name, state, stack, vmid, tailscale_ip, all service flags)
- History is stored under the new vmid when vmid changes

New endpoint: GET /api/instances/:vmid/history

The 'timestamps' section on the detail page is replaced with a
grid timeline showing timestamp | field | old → new for each event.
State changes are colour-coded (deployed=green, testing=amber,
degraded=red). Boolean service flags display as on/off.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

js/db.js

@@ -55,3 +55,8 @@ async function updateInstance(vmid, data) {
 async function deleteInstance(vmid) {
   await api(`/instances/${vmid}`, { method: 'DELETE' });
 }
+
+async function getInstanceHistory(vmid) {
+  const res = await fetch(`${BASE}/instances/${vmid}/history`);
+  return res.json();
+}